EXCHANGE 


THE  EFFECT  OF  THE  MAGNETIC  FIELD 
ON  THE  ABSORPTION  OF  X-RAYS 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL 

OF  CORNELL  UNIVERSITY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


BY 


JOSEPH  ADAM  BECKER 


Reprinted  from  PHYSICAL  REVIEW,  Vol.  XX,  No.  2,  August,  1922 


THE  EFFECT  OF  THE  MAGNETIC  FIELD 
ON  THE  ABSORPTION  OF  X-RAYS 


A  THESIS 

PRESENTED  TO  THE  FACULTY  OF  THE  GRADUATE  SCHOOL 

OF  CORNELL  UNIVERSITY  FOR  THE  DEGREE  OF 

DOCTOR  OF  PHILOSOPHY 


BY 

JOSEPH  ADAM  BECKER 


Reprinted  from  PHYSICAL  REVIEW,  Vol.  XX,  No.  2,  August,  1922 


; 


THE   EFFECT  OF  THE   MAGNETIC   FIELD   ON  THE 
ABSORPTION   OF  X-RAYS. 

BY  JOSEPH  A.  BECKER. 

SYNOPSIS. 

Effect  of  a  magnetic  field  on  the  absorption  of  x-rays,  if  it  can  be  measured,  would 
have  an  important  bearing  on  the  theory  of  atomic  structure.  Since  the  effect  is 
very  small,  a  differential  method  was  adopted  and  by  eliminating  various  spurious 
effects  the  apparatus  was  made  sensitive  enough  to  detect  a  change  of  i  part  in 
10,000.  Observations  were  made  with  a  peak  voltage  of  80  kv.  across  the  Coolidge 
tube  (mean  wave-length  about  0.3  A.)  and  with  a  magnetic  field  H  of  about  18,000 
gauss.  For  H  perpendicular  to  the  rays,  aluminum,  carbon,  copper,  iron,  nickel, 
platinum,  zinc,  and  silver  showed  changes  in  absorption  coefficients  of  +  8  ±  6, 
—  5.6  ±  2,  +  0.8  db  0.4,  —  10  ±  2,  +  1.6  ±  0.5,  +  1.7  ±  0.4,  —  1.2  ±  0.4  and 
+  1.6  ±  0.8  parts  in  10,000  respectively,  while  with  H  parallel  to  the  rays  the 
corresponding  changes  were  +  2.7  db  I,  +  3  ±  i,  +  1.4  ±  I,  —  0.5  ±  2,  +  0.7  ±  2, 
+  i.i  ±i  and  -f-  1.3  ±  i  X  io~4,  silver  not  being  tried.  These  results  are  in 
accord  with  the  hypothesis  that  the  magnetic  properties  are  largely  determined  by 
the  outer  shell  or  valency  electrons,  since  at  the  wave-lengths  used  by  far  the 
greater  part  of  the  absorption  is  due  to  the  inner  electrons.  Wood  was  also  tested 
with  softer  rays  having  a  mean  wave-length  of  about  1.2  A.,  and  showed  a  change 
of  +  80  ±  20  X  io~4  as  compared  with  3  X  io~4  for  carbon  for  wave-length  0.3  A. 
Following  the  suggestion  of  this  result  it  is  proposed  to  do  further  work  with  light 
elements  and  softer  x-rays. 

Differential  Method  of  Measuring  Small  Changes  of  Intensity  of  an  X-ray  Beam. — 
The  apparatus,  which  includes  two  similar  ionization  chambers  connected  so  that  the 
ionization  currents  nearly  neutralize  each  other,  is  described  in  detail  together 
with  the  precautions  necessary  to  eliminate  various  sources  of  error  and  to  attain 
the  sensitivity  mentioned  above. 

Bumstead  Electroscope. — A  simple  type  is  described.  Care  should  be  taken  to 
eliminate  convection  currents  within  the  instrument. 

PHE  following  investigation  was  undertaken  because  of  the  convic- 
•*-  tion  that  the  structure  of  the  atom  cannot  be  satisfactorily 
explained  without  taking  into  account  magnetic  forces.  It  was  hoped 
that  an  applied  magnetic  field  would  orient  the  ultimate  magnetic  particle 
and  that  this  shifting  would  result  in  a  measurable  change  in  the  absorp- 
tion of  x-rays.  A  study  of  this  effect  would  throw  a  new  light  on  the 
nature  of  the  ultimate  magnetic  particle  and  on  its  function  in  the  atom. 
This  search  is  not  new.  In  1914  and  1916  Dr.  Forman1  sought  for 
this  effect  in  iron.  In  his  second  paper  he  reports  an  increase  in  absorp- 
tion of  five  to  seven  parts  in  a  thousand  when  iron  is  magnetized  in  a 
direction  parallel  to  the  x-ray  beam.  Soon  thereafter  A.  H.  Compton2 

1  PHYS.  REV.,  3,  306-313,  1914  and  7,  119-124,  1916. 

1  J.  Wash.  Acad.  Sci.  8,  i,  1918  and  PHYS.  REV.,  14,  20-43  and  247-259,  1919. 


135 


JOSEPH  A.   BECKER. 


[SECOND 

LSBRI«S. 


developed  his  theory  of  scattering  and  later  stated  "that  the  effect 
observed  by  Forman  is  of  the  order  of  magnitude  to  be  expected  if  the 
electrons  are  rings  of  electricity  which  are  oriented  by  the  magnetic 
field."  1  This  led  him  to  look  for  a  change  in  y-ray  absorption  in  magne- 
tized and  unmagnetized  iron.  He  reports  negative  results  of  .023  ±  .018 
and  .004  ±  .01 9.3 

Since  1916  our  knowledge  of  x-ray  production  and  absorption,  as  well 
as  the  technique  of  control  and  measurement,  has  advanced  sufficiently 
to  warrant  a  repetition  of  Forman's  work  as  well  as  an  extension  to  other 
materials  than  iron. 

APPARATUS. 

The  method  used  is  essentially  a  differential  one  and  is  necessarily 
so  because  the  changes  sought  are  small. 

The  general  arrangement  of  apparatus  was  much  the  same  as  that 
used  by  Forman.  A  radiation- type,  tungsten-target  Coolidge  tube,  T,2 
is  practically  completely  enclosed  in  a  lead  box  L,  Fig.  I ,  and  sends  two 


$O      »!  All  Pi  men  sions   Ore  />    Cm. 


to/ts      ; 


K-/0-H 
detail  £ 


Fig.  1. 

beams  of  x-rays  through  three  pairs  of  preliminary  lead  slits,  through  the 
absorbers  A  and  A',  and  into  two  identical  ionization  chambers  Ii  and 
/2.  The  rods  RI  and  R%  are  connected  together  and  to  the  leaf  of  a 
Bumstead  electroscope  E.3  This  system  can  be  earthed  by  means  of  a 
mercury-break  key,  and  is  shielded  from  electrostatic  disturbances  by  a 
tin  shield  Sh.  The  casings  of  the  ionization  chambers,  as  well  as  the 
movable  plates  of  the  electroscope,  are  connected  to  the  terminals  of  a 
42o-volt  battery  of  dry  cells,  B,  whose  middle  point  is  earthed.  G  is  a 

1  PHYS.  REV.,  17,  38-41,  1921. 

2  Very  kindly  placed  at  the  disposal  of  Prof.  Richtmyer  by  Dr.  Coolidge. 

3  Am.  J.  of  Sci.,  32,  405-406,  1911  and  Phil.  Mag.,  22,  910,  1911. 


NoL>2XX']  ASBORPTION  OF   X-RAYS.  136 

glass  tube  connecting  the  two  chambers.  M  is  a  powerful  electromagnet 
of  Swiss  type.  The  diameter  of  the  face  of  the  pole-pieces  was  2  cm.  in 
the  first  part  of  the  investigation  and  4  cm.  in  the  second  part,  h  is  a 
holder  to  be  described  in  detail  later.  D  is  a  rectangular  opening  of  lead, 
the  upper  edge  of  which  is  attached  to  a  slow  motion  screw. 

The  procedure  is  as  follows:  one  specimen  is  placed  between  the  pole- 
pieces,  while  an  exactly  similar  one  is  placed  in  front  of  D.  The  size  of 
the  slit  at  D  is  varied  until  practically  no  net  charge  accumulates  on  the 
rods  RiRz  and  the  leaf  shows  either  a  small  rate  of  drift  or  none  at  all. 
The  magnet  is  energized.  If  the  absorption  of  the  specimen  in  the  field 
is  increased,  the  ionization  in  1%  will  decrease,  and  the  leaf  will  show  a 
change  in  the  rate  of  drift  (A  5)  where  5  is  the  rate  of  drift  in  divisions  per 
minute.  By  subsequently  determining  how  much  the  height  of  the  slit 
at  D  must  be  increased  to  produce  the  same  A5  we  can  compute  A///, 
the  proportional  change  in  the  intensity  of  the  transmitted  beam.  Then 
by  means  of  a  formula  to  be  developed  later  we  can  compute  A^//z,  the 
proportional  change  in  the  absorption  coefficient  due  to  the  field  H. 

Since  the  effect  is  small  it  is  necessary  to  be  able  to  keep  the  two 
ionization  currents  balanced  to  within  I  or  2  parts  in  10,000.  Con- 
sequently every  effort  was  made  to  reduce  the  errors  to  a  minimum. 
The  larger  part  of  the  time  was  taken  up  in  determining  and  eliminating 
spurious  changes  in  d.  The  following  list  gives  the  chief  factors  to  be 
guarded  against. 

SOURCES  OF  ERROR. 

(a)  Electrostatic  effects. 

(b)  Unsteadiness  of  leaf  of  electroscope. 

(c)  High-resistance  leaks. 

(d)  Stray  x-radiation. 

(e)  Slight  changes  in  the  position  of  the  focal  spot. 
(/)  Changes  of  current  through  the  tube. 

(g)   Peculiarities  of  the  ionization  chambers. 

(h)  Effect  of  the  magnetic  field  on  the  tube  or  ionization  chambers. 
(i)    Movement  of  the  specimen  caused  by  H  or  by  the  slight  shifting  of 
the  pole-pieces. 

To  show  how  these  errors  were  eliminated  it  is  necessary  to  give  a 
detailed  description  of  the  apparatus. 

THE  ELECTROSCOPE. 

Fig.  i  shows  a  detail  drawing  of  the  electroscope.  It  can  be  con- 
structed by  an  amateur  mechanic  in  a  short  time.  The  brass  box  was 
made  detachable,  but  the  joints  were  sealed  with  wax  to  make  them 


rSECOKD 

137  JOSEPH   A.    BECKER.  [SERIES. 

air  tight.  The  plates  and  the  leaf  are  adjustable.  Sulphur  insulation 
was  found  convenient  and  satisfactory.  The  position  of  a  definite  point 
on  the  gold-leaf  was  observed  through  a  low-power  microscope  with  a 
5O-division  scale  in  the  ocular.  These  50  divisions  correspond  to  approxi- 
mately 2  mm.  At  first  considerable  difficulty  was  encountered  in  keeping 
the  grounded  leaf  at  rest.  The  trouble  was  finally  ascribed  to  heat 
convection  currents,  and  was  eliminated  by  completely  enclosing  the 
instrument  with  a  cardboard  housing.  The  working  sensitivity  of  the 
leaf  was  about  80  div.  per  volt  on  the  leaf.  This  sensitivity  could  easily 
have  been  increased  four  or  five  fold  by  either  lowering  the  leaf,  moving 
both  plates  closer,  or  by  increasing  the  voltage  on  the  plates.1  At  the 
high  sensitivities  it  becomes  somewhat  more  difficult  to  keep  the  position 
of  the  leaf  steady.  Unsymmetrical  changes  of  voltage  on  the  plates 
were  about  one  fifth  as  effective  in  deflecting  the  leaf  as  the  same  voltage 
on  the  leaf,  e.g.,  if  the  ground  is  shifted  one  volt  the  resultant  deflection 
is  the  same  as  if  the  leaf  had  been  charged  to  .2  volt.  This  fact  necessi- 
tated keeping  the  relative  voltages  on  the  plates  constant  to  within  10 
millivolts.  When  not  too  old,  the  dry  cells  proved  satisfactory  so  long 
as  their  temperature  was  kept  constant. 

The  drift  in  the  leaf  due  to  change  of  plate  voltage,  high  resistance 
leaks,  and  electrostatic  disturbances  was  seldom  more  than  5  divisions 
in  half  an  hour.  Neither  was  the  rate  of  drift  increased  by  running  the 
x-ray  tube  with  the  openings  at  h  and  D  closed  by  small  lead  plates.  This 
shows  that  the  first  four  sources  of  error  mentioned  above  were  effectively 
eliminated.  To  accomplish  this  it  was  found  essential  completely  to 
inclose  the  tube  with  lead  and  put  up  the  preliminary  screens. 

Sources  of  error  e,  /,  and  g  were  found  by  repeatedly  observing  that  5 
would  change  radically  whenever  the  current  through  the  tube  changed. 
At  this  time  the  high  tension  for  the  tube  was  furnished  by  a  G.  E.  x-ray 
transformer  while  the  filament  was  supplied  from  a  step-down  trans- 
former. Even  though  the  primary  of  both  transformers  was  supplied 
from  a  rotary  converter  (D.C.  to  A.C.)  whose  voltage  could  be  kept 
constant  to  within  2  per  cent.,  yet  the  current  through  the  tube  would 
vary  as  much  as  50  per  cent.  Ideally  this  should  not  have  disturbed  a 
balance  since  both  beams  should  be  affected  equally.  Practically  it  did 
and  the  only  satisfactory  way  of  keeping  the  current  steady  was  to 
light  the  filament  by  means  of  a  12-volt  storage  battery  in  the  high- 
tension  circuit.  The  filament  current  was  varied  by  means  of  a  slide- 
resistance  operated  by  an  insulated  handle.  Even  with  this  arrangement 
the  contacts  in  the  filament  circuit  were  gone  over  periodically  to  insure 
steady  currents. 

1  Dadourian,  PHYS.  REV.,  14,  238,  1919,  obtained  a  working  sensitivity  of  5,000  divisio  ns 
per  volt. 


VOL.  XX.1 
No.  a. 


ABSORPTION  OF  X-RAYS. 


'3* 


In  all  Figs,  the  values  of  5  for  H  off  are  represented  by  • ;  for  H  on,  by  an  x.  H  =  approx. 
18,000  gauss. 

In  Figs.  2  to  16,  H  was  perpendicular  to  the  x-rays;  the  voltage  was  80  kv.  peak;  a  -f  A5 
means  a  decrease  in  I,  or  an  increase  in  ju. 


139 


JOSEPH  A.  BECKER. 


[SECOND 

[SERIES. 


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In  Figs.  17  to  24,  H  was  parallel  to  X\   the  voltage  was  80  kv.  peak;  a    +  A5  means  an 
increase  in  I  or  a  decrease  in  ju. 

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VoL.^XX.J  ABSORPTION  OF  X-RAYS. 

A  pin-hole  picture  of  the  focal  spot  showed  that  it  consisted  of  a  ring 
7/1 6  inch  in  diameter  with  two  irregular  small  areav=  inside.  Conse- 
quently if  the  first  slit  5  is  narrow  and  the  focal  spot  for  any  reason  shifts, 
even  though  the  shift  is  so  slight  as  to  be  imperceptible,  a  slightly  different 
area  becomes  effective  in  illuminating  the  slit  at  D  and  thus  the  balance 
is  destroyed.  The  preliminary  slits  were  therefore  made  3/4  inch  wide 
while  the  final  slits  at  D  and  h  were  only  1/8  to  1/4  inch  wide.  In  this 
way  every  point  of  the  final  slit  received  radiation  from  every  point  of 
the  focal  spot  even  though  the  focal  spot  did  shift  slightly.  Incidentally, 
widening  the  preliminary  slits  also  increased  the  available  energy.  The 
improvement  brought  about  by  this  change  show?  up  clearly  by  con- 
trasting the  steadiness  in  Figs.  2  and  14  for  narrow  slits  with  Figs.  3 
and  13  for  the  \videned  slits. 

The  ionization  chambers  were  made  of  brass  tubing  and  were  fitted 
with  square  ends.  They  were  mounted  on  sulphur  supports.  The  rods 
were  supported  and  insulated  by  sulphur  plugs  containing  the  usual 
earthed  guard  rings.  The  chambers  were  filled  with  methyl  bromide. 
Unfortunately  this  gas  in  the  presence  of  moisture  reacts  with  zinc  and 
copper  thus  making  it  necessary  to  refill  \he  chambers  every  three  or 
four  weeks.  When  doing  so,  care  was  taken  to  fill  both  chambers  to  the 
same  pressure,  otherwise  a  change  in  the  tube  current  of  .1  milliamp 
might  unbalance  the  ionization  in  the  chambers  by  several  parts  in  1,000. 
Later  the  two  chambers  were  connected  by  a  glass  tube. 

During  the  course  of  the  investigation  it  was  noticed  that  the  balance 
was  particularly  sensitive  to  slight  shifts  in  the  rear  end  of  the  ionization 
chambers.  A  further  study  revealed  the  fact  that  an  appreciable  per- 
centage ot  the  ionization  was  caused  by  photoelectrons  emitted  by  the 
rear  end  of  the  chamber.  Materials  of  low  atomic  weight  were  found  to 
cause  a  smaller  increase  in  the  ionization  than  those  of  high  atomic  weight. 
The  arrangement  decided  upon  as  least  troublesome  was  an  aluminum 
plate  coated  on  the  inside  with  wax. 

SPURIOUS  EFFECTS  DUE  TO  THE  MAGNETIC  FIELD  H. 

Forman  found  that  the  stray  field  of  his  electromagnet  deflected  the 
electron  stream  in  his  tube.  He  finally  remedied  the  difficulty  by  means 
of  compensating  coils  and  iron  screens.  The  magnet  in  this  present 
investigation  was  placed  at  a  considerable  distance  from  the  tube.  Its 
stray  field  near  the  tube  was  less  than  the  earth's  field  and  \vhen  possible 
was  opposed  to  it.  The  final  test  as  to  whether  the  stray  magnetic 
field  affects  either  the  tube  or  the  ionization  is  to  make  blank  runs  either 
with  no  absorbers  or  with  absorbers  in  front  of  the  magnet.  Several 


rSECOND 

,     JOSEPH  A.   BECKER.  [SERIES. 

such  runs  were  made  and  in  no  case  was  any  effect  greater  than  2  parts 
in  10,000  obtained.     (See  Fig.  18). 

Several  times  large  apparent  changes  in  absorption  were  noted  when 
the  specimen  was  between  the  pole-pieces,  but  in  all  such  cases  it  could 
be  shown  that  either  the  field  distorted  the  specimen  or  the  shifting  of 
the  pole-pieces  displaced  the  specimen.  This  was  particularly  true  in 
the  case  of  iron  and  nickel,  and  with  thin  specimens.  Fig.  16  shows  a 
spurious  change  of  about  2\  parts  in  1,000.  Due  to  the  high  absorption 
coefficient  for  platinum  the  spec  imen  was  only  I  or  2  mils  thick.  Small 
forces  might  easily  distort  the  thin  specimen.  When  this  same  specimen 
was  pasted  on  a  thin  uniform  strip  of  wood  and  another  run  taken,  as 
shown  in  Fig.  15,  AI  is  only  a  few  parts  in  10,000.  Practically  the  same 
result  was  obtained  when  the  specimen  was  tightly  clamped  in  the  holder. 

THE  SPECIMEN  HOLDERS. 

Two  holders  hi  and  h%  were  designed  and  are  shown  in  detail  in  Fig.  I. 
hi,  which  was  used  when  H  was  perpendicular  to  the  rays,  consists  of  a 
brass  collar  with  bevelled  edges  which  match  the  pole-pieces.  Two 
screws  fasten  the  specimen  to  two  ledges  at  opposite  ends  of  a  diameter. 
Perpendicular  to  this  diameter  are  openings  to  allow  the  beam  to  pass. 
This  holder  may  not  be  as  reliable  as  the  accuracy  of  the  remainder  of 
the  outfit  warrants.  Consequently  a  new  one  has  been  designed  which, 
it  is  hoped,  will  give  more  reliable  results. 

Holder  h2  was  carefully  designed  and  constructed.  It  was  used  when 
H  was  parallel  to  the  rays.  The  specimen  is  firmly  held  in  front  of  the 
lead  opening  by  means  of  screws  passing  through  the  brass  cover.  The 
pole-pieces  are  firmly  screwed  up  against  the  holder. 

THE  FINAL  SLIT  AT  D. 

The  opening  at  D  was  usually  3/16  inch  wide  and  about  1/2  inch  high. 
A  balance  was  obtained  by  changing  the  height  by  means  of  the  microm- 
eter screw.  One  turn  corresponds  to  i/ioo  inch  and  there  are  100 
divisions  on  the  drum.  Thus  with  a  slit  1/2  inch  high,  5  drum  divisions 
($D)  correspond  to  a  change  in  area  of  i  part  in  1,000,  i.e.,  A/i,ooo.  By 
noting  d  for  a  series  of  positions  of  the  drum,  A 5  for  loD,  or  else  A 5  for 
^4/i,ooo  can  be  computed.  The  uniformity  of  intensity  of  the  x-ray 
beam  over  the  whole  opening  was  shown  by  the  fact  that  this  ratio  was 
independent  of  the  height  of  the  opening. 

In  all  measurements  of  large  rates  of  drift,  say  greater  than  60  divisions 
per  minute,  it  was  found  necessary,  in  order  to  get  consistent  results,  to 
apply  a  correction  to  the  observed  8.  Table  I.  gives  the  values  of  the 


VOL.  XX. J  ABSORPTION  OF   X-RAYS. 

corrections  to  be  added  to  the  absolute  value  of  the  observed  8.  These 
corrections  were  obtained  by  noting^  for  a  number  of  openings  differing 
by  equal  small  steps  and  plotting  5  against  the  opening.  Instead  of  a 
straight  line,  a  curve  was  obtained  which  deviated  more  and  more  from 
the  straight  line  as  5  increased.  The  deviations  gave  the  corrections  to 
be  added.  This  lag  is  probably  due  to  air  friction  and  is  a  function  of  the 
dimensions  and  shape  of  the  leaf  and  the  air  pressure.  It  is  analogous 
to  the  lag  in  a  quadrant  electrometer. 

TABLE  I. 

Observed  5 50  100  150  200  250  300  400 

Correction 5  2.5  9  19  37  60  90 

SENSITIVITY  OF  APPARATUS. 

To  show  how  sensitive  the  apparatus  is,  one  need  only  obtain  a  balance, 
and  then  interpose  a  very  thin  sheet  of  paper.  After  a  few  seconds1 
lag  the  leaf  begins  to  drift  and  continues  to  do  so  until  a  few  seconds 
after  the  paper  is  removed.  A  few  degrees  change  in  temperature  in 
the  slit  D  can  be  detected;  the  slight  "give"  in  the  floors  caused  by  a 
person  walking  past  the  supports  for  the  slit  D  produces  an  observable 
effect ;  air  currents  also  cause  disturbances.  Under  these  circumstances 
it  seems  almost  hopeless  to  try  to  measure  an  effect  less  than  one  part  in 
1,000  with  an  accuracy  better  than  10  per  cent.  However,  if  the  tem- 
perature and  current  could  be  maintained  constant  and  if  mechanical 
disturbances  were  minimized,  the  apparatus  could  detect  changes  as 
small  as  2  or  3  parts  in  100,000. 

COURSE  OF  A  COMPLETE  RUN. 

The  following  illustrates  the  course  of  a  complete  run. 
Voltage  across  tube :  80  kv.  peak. 
Sensitivity  of  leaf:  44.2  div./volt. 

1.  Steady  test:  x-rays  shut  off  by  lead  plates.     Position  of  leaf  every 
15  seconds: 

25.2  25.2  25.0  25.0  24.9  24.9  24.9  24.8  24.8  24.8  24.7  24.7 

24.7  24.5  24.4  24.2  24.2  24.2  24.2  24.2  24.1  24.0  24.0  24.0 

24.0  24.0  23.9  23.8  23.9  23.9  23.9  23.8  23.7  23.6  23.6  23.7 

23.7  23.7  23.7  23.6 

2.  Sensitivity  determination  with  no  absorber  in. 
Size  of  opening  =  3,900  drum  divisions  or  3,900  D, 

Drum  scale  20  up  5  =  —  40  X  60/11.4  =  —  210  Corrected  5  =  —  230, 
Drum  scale  25  up  5  =  —  23.3  X  60/30  =  —  46.6  Corrected  8  =  —  47, 
Drum  scale  30  up  8  =  40  X  60/18.9  =  127  Corrected  5  =  +131* 


143  JOSEPH  A.   BECKER.  \jSSSn. 

Ad/ioD  =  1 80  X  2  =  360  for  2.2  milliamps  =  82  per  milliamp, 
A5  for  4/1000  =  A5/io£>  X  3-9/IO  =  140. 

3.  Steady  test  with  x-rays  entering  chambers.     Readings  of  leaf  every 
15  seconds: 

19.0     19.5     18.5     19.5         20.0     20.7     19.5     18.5          19.7     21.0     22.0     21.0 
21.0     21.5     21.8     22.5         24.0     23.8     24.0     25.0         25.3     25.0     24.0     24.0 

4.  Sensitivity  with  iron  absorber  in.     Opening:  3800^. 

Drum  40  up  5  =  —  40  X  60/15  =  —  160  Corrected  5  =  —  170, 
Drum  60  up  5  =  —  20  X  60/13  =  —  92  Corrected  5  =  —  94, 
Drum  80  up  5  =  —  12  Corrected  5  =  —  12, 

A6/IO-D  =  (76  +  82)/2  X  2  =  39.5  for  1.3  milliamps  =  30.4  per  milliamp, 

T/T    _  A5/IO-D  for  Fe  absorber  _  . 

=  M/ioD  for  no  absorber  ~  3°4/8' 
A5  for  4/1000  =  (39-5/io)  X  3-8  =  15. 

5.  Effect  of  magnetic  field  on  absorption  of  iron.     H  =  18,000  gauss. 
H  is  put  on  for  one  minute  every  other  minute  starting  with  the  second. 
The  position  of  the  leaf  is  read  every  15  seconds. 


33.0 


A  positive  A  5  means  an  increase  in  absorption,  or  a  decrease  in  /. 

From  this  data  the  average  5  for  each  minute  was  computed  and  plotted 
in  the  latter  half  of  Fig.  3. 

This  figure  shows  a  change  of  d  due  to  H  equal  to  —  13  div.  Since 
AS  for  4/iooo  =  15; 

_       -  *3       _          8-7 


a   \j 

Switch 

Switch 

closed 

opened 

32.8 

33.0 

34.0 

33.0 

31.5 

29.2 

25.5 

24.5 

26.0 

28.0 

29.7 

30.8 

29.0 

26.0 

25.0 

24.8 

26.2 

30.0 

31.2 

34.0 

32.5 

30.0 

28.3 

28.5 

28.8 

29.5 

30.5 

30.5 

29.5 

29.0 

27.5 

30.0 

31.0 

34.0 

35.8 

36.5 

34.0 

32.0 

30.0 

29.0 

31.0 

33.0 

35.5 

38.0 

I  15  X  1000          10,000 

From  a  formula  developed  below 

AM  =  I  AJ  =  t  +8.7        -  8.8 

M  loge(/0//)      /  10,000       10,000 

Consequently  a  field  of  18,000  gauss  decreases  the  absorption  coefficient 
of  iron  by  8.8  parts  in  10,000. 


VoL^XX.J  ABSORPTION  OF  X-RAYS.  144 

DEVELOPMENT  OF  FORMULAE. 
As  is  well  known, 

I  =  Jo*-*'.  (l) 

If  fj,  can  be  made  to  change  in  a  given  specimen,  then 


or 

A/ 

From  (i) 

A7 


_  N/ 

.  .  —    -        ~  A 


V  1  •      •  j- 

M  /  M 

or 

AM  _  i          A7 

M   =  "log  (I,/ 1)'  I  ' 

Given:  70,  M>  and  AM  for  a  certain  material.     What  is  the  best  thickness 
to  use  so  that  AI  shall  be  a  maximum? 
From  (2) 

A/=  - 
For  a  maximum 


or 

-  I0A/i(e-"«  -  Mte~M)  =  o 

Since  J0,  A/A,  and  e~M*  are  not  zero, 

I  —  [it  —  o        or        ju/  =  i 
or 

loge  Jo/7  =  i,         Jo/J  =  2.73;         J//o  =  .368. 

That  is  AJ,  which  determines  A5,  will  be  greatest  if  t  is  so  chosen  that 
J//o  =  -368. 

RESULTS. 

Figs.  2-1 6  show  the  results  obtained  with  H  perpendicular  to  the  path 
of  the  x-rays;  Figs.  17-26,  the  results  for  H  parallel  to  the  x-rays.  For 
all  but  the  last  two  figures  the  peak  voltage  across  the  tube  was  about 
80  kilovolts,  which  corresponds  to  a  minimum  wave-length  of  .15  A. 
The  maximum  energy  was  probably  in  the  neighborhood  of  .3  A.  Each 
figure  contains  the  date  on  which  the  data  were  obtained,  the  values  of 


(SECOND 

145  JOSEPH  A.   BECKER.  [SERIES. 

A///,  AM/JU,  and  AS  for  ^4/iooo.  It  is  important  to  take  this  last-named 
factor  into  account  when  comparing  the  curves. 

A  comparison  of  the  steadiness  of  this  apparatus  with  that  attained  by 
Forman  is  given  in  Fig.  25  which  shows  Forman's  results  for  iron  with 
81  kv.  (R.M.S.)  across  the  tube,  and  the  present  result  for  iron  under 
similar  conditions,  both  plotted  to  practically  the  same  scale. 

Figure  26  shows  one  of  two  preliminary  runs  with  a  peak  voltage  of 
27  kv.  across  the  tube.  A  previous  run  on  wood  at  the  higher  voltage 
had  revealed  a  A/*//*  of  4  or  5  parts  in  10,000.  At  the  lower  voltage  the 
sensitivity,  consistency,  and  reliability  are  considerably  less  but  still 
AJU/M  is  clearly  about  8  parts  in  1,000  or  about  16  times  as  large  as  before. 
This  would  indicate  that  larger  changes  may  reasonably  be  looked  for 
at  longer  wave-lengths  with  low  atomic  weight  elements.  This  clue, 
it  is  hoped,  can  be  followed  up  in  the  next  few  months. 

The  results  show  that  while  the  magnetic  field  does  change  the  absorp- 
tion of  x-rays  in  various  materials  the  effects  for  wave-lengths  near  .3  A. 
are  small.  At  this  wave-length  the  largest  effect  is  shown  by  iron  for 
which  A/Z/JLI  is  about  I  part  in  1,000  when  H  is  perpendicular1  to  X. 
For  H  parallel  to  X,  Fig.  19  shows  that  the  change  is  much  smaller,1 
which  may  be  partly  due  to  the  large  demagnetizing  field  for  such  a  thin 
sheet  of  iron.  Of  the  other  materials  examined  carbon  and  aluminum 
show  the  largest  change.  Nickel,  platinum,  copper,  zinc  and  silver 
show  changes  smaller  than  3  parts  in  10,000  for  either  direction  of  H. 
At  a  mean  wave-length  in  the  neighborhood  of  1.2  A.,  wood  shows  a 
change  in  absorption  of  about  8  parts  in  1 ,000  for  H  parallel  to  x-rays. 

The  following  list  sums  up  the  values  of  Aju//*2  together  with  an 
estimated  probable  error.  The  conditions  under  which  each  value  was 
obtained  can  be  found  in  the  figures  or  in  the  legend  for  the  figures.  The 
estimate  of  the  probable  error  i?  based  on  the  oscillations  of  5,  the  value 
of  A//I  due  to  H  for  no  absorber,  and  on  the  author's  experience  and 
judgment  of  the  likelihood  of  small  spurious  effects. 

All  results  are  expressed  in  number  of  parts  change  in  10,000,  a  plus 
sign  meaning  an  increase  in  /*. 

For  H  perpendicular  to  X  rays: 

Iron  -  10  ±  2,  Nickel  +  1.6  ±  0.5,  Platinum  +  1.7  ±  0.4, 

Copper  +  0.8  ±  0.4          Zinc  -  1.2  ±  0.4,  Silver  +  1.6  db  0  8, 

Carbon  —  5.6  ±  2,  Aluminum  +  8  ±  6. 

1  Opposite  to  Forman's  results. 

2  Corrected  for  a  small  effect  observed  with  no  absorber,  which  was  never  greater  than  2 
parts  in  10,000. 


OL.^.  ABSORPTION  OF   X-RAYS. 

For  H  parallel  to  X  rays : 

Iron  -  0.5  ±  2,  Nickel  +  0.7  ±  2,       '"  Platinum  +  1.1  =fc  1, 

Copper  +  1.4  ±  1,  Zinc  +  1.3  ±  1,  Carbon  +  3  ±  1, 

Aluminum  +2.7  ±1,       Wood  (longer  wave-lengths)  +  80  ±  20. 

DISCUSSION  OF  RESULTS. 

These  results  are  in  accord  with  the  hypothesis  that  magnetic  properties 
are  largely  conditioned  by  the  outer  atomic  shell  or  valency  electrons. 
If  the  magnetic  fields  inside  the  atom  are  actually  as  large  as  is  supposed, 
we  cannot  hope  to  penetrate  deeply  into  the  atom  with  external  fields. 
Consequently  we  should  hardly  expect  to  produce  any  changes  in  the 
inner  shells.  Now  it  is  generally  believed  that  the  K  absorption  and 
emission  are  associated  with  the  innermost  shell  while  the  L  and  M  radia- 
tions are  due  to  the  next  two  shells  of  electrons  respectively.  Professor 
Richtmyer1  has  shown  that  very  probably  for  wave-lengths  less  than  the 
Ka  limit,  by  far  the  larger  part  01  the  abrorption  is  used  up  in  exciting 
the  K  fluorescent  radiation,  a  small  percentage  of  the  energy  absorbed, 
for  example,  let  us  say  5  per  cent.,  excites  the  L  fluorescence,  and  pre- 
sumably even  a  much  smaller  percentage,  let  us  say  .5  per  cent.,  goes  to 
excite  the  M  fluorescence.  On  the  above  hypothesis  we  should  expect 
to  be  unable  to  affect  the  K  and  L  absorption  of  all  substances  having 
three  or  more  shells  of  electrons.  To  be  specific,  let  us  consider  aluminum 
which  on  Langmuir's  theory  contains  a  K  shell  of  2  electrons,  an  L  shell 
of  8  electrons,  and  an  outer  shell  of  3  electrons.  If  the  external  magnetic 
field  produces  no  change  in  the  K  and  L  absorption  but  as  much  as  a  10 
per  cent,  change  in  the  absorption  due  to  the  outer  shell,  the  change  in 
the  total  absorption  would  amount  to  only  .10  X  .005  or  5  parts  in  10,000. 
On  the  other  hand,  carbon  which  contains  only  2  shells  would  on  the 
same  suppositions  show  a  change  in  the  total  absorption  equal  to 
.10  X  .05  or  5  parts  in  1,000.  We  should  also  expect  a  larger  change  in 
the  total  absorption  if  the  wave-length  were  longer  than  the  Ka  limit. 
Both  of  these  expectations  are  supported  by  the  fact  that  at  a  given 
wave-length  carbon  and  aluminum  show  larger  changes  than  the  heavier 
elements  (excepting  possibly  iron  and  nickel),  and  that  wood  shows 
larger  changes  at  long  than  at  short  wave-lengths. 

On  Compton's  theory  of  scattering  we  might  look  for  an  appreciable 
change  in  that  part  of  the  absorbed  energy  which  is  scattered.  In  the 
low  atomic  weight  elements  a  large  percentage — as  much  as  50  per  cent. 
• — of  the  absorbed  energy  goes  into  scattered  radiation.  This  investiga- 
tion shows  that  the  change  in  the  scattered  radiation  cannot  exceed 
about  i  part  in  1,000. 

1  PHYS.  REV.,  18,  13-30,  1921. 


147  JOSEPH  A.   BECKER. 

The  most  hopeful  region  to  work  in  seems  to  lie  near  the  absorption 
limit  of  the  outermost  shell.  The  most  promising  materials  are  the 
ones  in  the  first  two  groups  of  the  periodic  table.  While  the  experimental 
difficulties  in  working  in  the  neighborhood  of  100  A.  are  great,  the  author 
believes  that  it  will  be  well  worth  the  effort  and  hopes  to  have  the  oppor- 
tunity of  continuing  the  search  in  that  direction. 

In  conclusion  the  author  wishes  to  express  his  appreciation  and 
gratitude  for  the  ever  helpful  and  never  failing  encouragement,  inspiration 
and  assistance  received  from  Professor  Richtmyer. 

CORNELL  UNIVERSITY, 
ITHACA,  NEW  YORK, 
March,  1922. 


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